Her brother threatened to kill her three times, but Islam Bibi loved her job as the top female police officer in Helmand so much that she defied both his demands she stay home and intimidation from Taliban, and kept putting on her uniform and going to work.

The 37-year-old mother of three was an embodiment of how far a few brave women have come in conservative southern Afghanistan, but on Thursday her job apparently cost Bibi her life.

Unidentified men gunned her down at the start of her morning commute, which she did without bodyguards or any other protection, riding pillion on the back of her son-in-law's motorbike.

"They were both injured, police took them to hospital and after 45 minutes she passed away. Her son-in-law is still being treated," said Omar Zwaak, a spokesman for the provincial governor.

The 7am (0330 BST) hit was a symbolic strike against women's rights in a province that has for years been an insurgent stronghold, and has seen particularly high levels of violence this year as the withdrawal of Nato troops gathers pace.

An intelligence source said there were hundreds more insurgents flooding into the province for the summer "fighting season" than arrived last year, and in late May more than 100 tried to overrun government outposts in northern Sangin district. They were repelled, but only after several days' heavy fighting.

The violence is also creeping into the provincial capital of Lashkar Gah as insurgents target those they see as opposed to their values.

"I can't go into the city alone because of security issues," said Abdullah Baluch, who works on children's rights for the Afghan Independent Human Rights Commission. "Security is really getting worse in Helmand these days, especially for me because all the Taliban know who I am."

Bibi worked in Lashkar Gah, heading a team of female officers in the criminal investigation department, a job that most of her family were implacably opposed to. "My brother, father and sisters were all against me. In fact my brother tried to kill me three times," she told the Sunday Telegraph earlier this year. "The government eventually had to take his pistol away."

After years of recruiting drives, women still make up less than 2% of Helmand's police force. The opposition of Bibi's family is typical in southern Afghanistan. Many consider it shameful for women to work outside the home, where they may meet men from outside their family.

The Taliban has also run a campaign of intimidation and assassination against both working women and government officials, making female officials particularly vulnerable.

In 2008 the Taliban killed Malalai Kakar, the head of the department of crimes against women in nearby Kandahar city and at the time the most senior female police officer in the country. She was shot dead on her way to work, and left behind six children.

Two years earlier the provincial head of women's affairs for Kandahar, the Taliban's birthplace, was killed. Last year two women who held the same post in eastern Laghman province were shot dead within six months.

"It has been increasingly dangerous over recent years to be a woman in public life in Afghanistan, and there has been a growing body count of women who have been brave enough to ignore the risks," said Heather Barr of Human Rights Watch.

"With the withdrawal of international forces and the deterioration we are seeing in women's rights, there is every reason to fear that these dangers will become even worse in the years ahead, especially in provinces such as Helmand that remain deeply insecure."

Human Rights Watch said this year that female police officers were often subject to sexual harassment and abuse from their colleagues, in part because they lacked even basic facilities. There are just a handful of female toilets in all the police stations of Afghanistan and women using male latrines are particularly vulnerable, the group said.